1. Field of the Invention
The present invention relates to a box type structural body containing a lap joint to which welding can not be applied from the inner side of the body, and a vehicle end sill which corresponds to such box type structural body.
2. Description of the Related Art
A structural body for a head portion of a (commuter) railway vehicle (rolling stock) is shown in FIG. 4. In an example in FIG. 4, a bone member, which is located at a front end (or rear end) of a vehicle A and is extending along the vehicle width direction, is called an "end sill". An end sill 1 is provided to bear the load applied when the vehicle A collides with the other vehicle or the object. The end sill 1 is a box type structural body which has, for example, a longitudinal sectional shape as shown in FIG. 5A.
The end sill 1' shown in FIG. 5A is formed as a box shape by extending a beam-like end member 11 such as channel member at a front end portion of the vehicle A along the width direction. Upper and lower face plates 12, 13 are welded to upper and lower surfaces of the end member 11. The box type structure is a closed (or almost closed) structure and has no inner space into which an operator or a welding machine can enter. Therefore, lap joints W' between the end member 11 and the upper and lower face plates 12, 13 are welded merely from the outer side of the box type structure and are not welded from the inner side of the box type structure. A weld seam W' of the joint W' extends linearly, as shown in FIG. 5B. The weld seam Wa' of the joint W' is positioned perpendicularly to the direction of the load applied from the left side in FIG. 5B, which is caused by a collision of the vehicle A with the other object (such as the other vehicle).
The face plates 12, 13 at the end sill 1' shown in FIGS. 5A to 5C can be deformed in the form of an out-of-plane deformation by low load, and in some cases they are broken off since the bending moment is applied to the joint W' correspondingly. When the load caused by the collision is applied from the left side of FIG. 5A with the result of an application of the bending moment onto the joints W' (see an arrow shown in FIG. 5D), such bending moment is received only by a throat thickness of a weld bead of the weld seam Wa' since the weld seam Wa' of the joint W' is formed as a straight line. For this reason, the joint W' tends to deform as shown in FIG. 5D and also tends to break of f as shown in FIG. 5E. In this manner, if the joint W' is easily deformed and broken off, a collision energy cannot be sufficiently absorbed by the end sill 1' which includes the face plates 12, 13, etc.
As for the joint W' in the end sill 1' as shown in FIGS. 5A to 5E in the related art, FIG. 5F is a graph showing a typical relation between an amount of deformation .delta. and an reaction P of the joint W' when the joint W' is deformed. Since the joint W' is broken off as shown in FIG. 5E at a point x of time when the reaction P is increased, the succeeding reaction P is reduced abruptly. An amount of collision energy which can be absorbed by the joint W' can be expressed by a shaded area (i.e., .intg.Pd .delta.) in FIG. 5F. As shown in FIG. 5F, the amount of absorbed collision energy is not sufficiently large. If the absorption energy of the joint W' is small, the energy which can be absorbed by the end sill 1' is also small. Therefore, remaining collision energy, which can not be absorbed by the end sill 1', acts onto other portions of the vehicle A and thus the considerable damage of the vehicle A is caused.
Although above-mentioned explanation has been made by taking the vehicle end sill into consideration as an example, the similar problem also occurs in other box type structural bodies. In other words, if the box type structural body includes the lap joints which are linearly welded only from the outer side (one side welding) for the structural reason and the direction of the weld seams of the lap joints is set perpendicular (or closely perpendicular) to the direction of the load, the lap joints can be broken off easily when they receive the load. In addition, in the end sill 1' in FIG. 5A and the box type structural body in the related art, another fracture mode may be considered. That is a shear fracture of the weld bead in the form that the end member 11 and the face plates 12, 13 are shifted in parallel with their lapped faces without the deformation shown in FIGS. 5D and 5E.